CHICAGO (WBBM/STMW) — A 12-year-old boy was shot in the back during dismissal in front of a Gresham neighborhood elementary school Tuesday afternoon on the South Side.

The argument started on Facebook, witnesses and relatives of the 12-year-old victim told the Sun-Times.

But by 2.45 p.m. Tuesday, the alleged online threats became terrifyingly real when a 14-year-old gunman brazenly opened fire outside a South Side elementary school as hundreds of children were dismissed for the day.

Witnesses dived for cover as the teen-age gunman — wearing a brown Hollister brand hoodie — shot his 12-year-old gang rival in the back with a revolver outside Cook Elementary School in the 8100 block of South Bishop, witnesses say.

“We were coming out of the front door of the school when we saw him,” student Rashaya Williamson, 13, said, “Suddenly he started shooting and everyone was screaming and running.”

The victim was in stable condition Tuesday evening at Christ Hospital in Oak Lawn, police said. After he was shot, he ran back towards the school “but the door was locked and he couldn’t get in,” Williamson said.

Eventually somebody let him in as he cried in agony, while the gunman ran away southbound on Bishop, several witnesses said.

Police Cmdr. Eddie Johnson said detectives had spoken with many “really good witnesses,” and had a good idea of the suspect’s identity. He bemoaned the shooter’s “total disregard for human life.”

The victim’s relatives said the shooter was a former Cook student. He had fought with the victim previously, they said.

Police could not confirm that he turned himself in Tuesday evening.

The threats the shooter made on the victim’s Facebook profile were deleted earlier Thursday, the victim’s family say, and they could not be confirmed by the Sun-Times, though the victim’s friends were already using his page to vow bloody revenge Tuesday evening.

One post on the victim’s Facebook said they would “ride back down” on the shooter and his friends.